THE DEVONIAN PERIOD. 457 



relatively, but they were not abundant. The other echinoderms 

 were still less important. 



The corals, doubtless for like reasons, were not generally abundant, 

 though Favosites was well represented, as were also a few other forms. 

 The genus Michelinia (Fig. 206, s), which became abundant in the 

 Onondaga and Hamilton epochs, makes its first appearance here. 



Minor forms. — There were hydrozoans, among which Stromato- 

 pora became so common as to form, almost alone, beds three or four 

 feet thick; a few graptolites, which were about to become extinct; 

 some sponges, and doubtless not a few other forms that have escaped 

 fossilization. 



The notable absence of fishes. — It is to be especially noted that fish 

 remains are almost or quite absent from the Helderberg deposits, although 

 they had appeared in the epoch just preceding and reappeared in great 

 abundance and in remarkable forms in the Ononclagan epoch that 

 soon followed. However their absence is to be explained — and it 

 seems to be nearly inexplicable — it serves to strengthen the belief 

 that the Onondaga fauna originated in a tract well isolated from the 

 originating tract of the Helderberg fauna. 



The Oriskany Fauna. 



The key to the interpretation of the typical Oriskany fauna appears 

 to lie in the fact that it was a sand-loving assemblage of species and 

 that its habitat was the sand-flats that skirted the ancient shore belt. 

 There was a minor phase of the fauna found in calcareous deposits. 

 The fauna was bound by many close ties to the Helderberg, especially 

 that phase of it which is present in the more calcareous beds, but it 

 contained also distinct features implying in part a separate origin. 

 However originated, it was so constituted as to fit the conditions of 

 shallow water and sandy bottoms, while the Helderberg fauna was 

 adapted to calcareous and muddy bottoms, and perhaps to somewhat 

 greater depths. The Oriskany fauna, as we know it, was also later 

 than the Helderberg, which means that in so far as it was not derived 

 from the Helderberg, it had been developing longer since the two 

 diverged from a common stock, and should therefore be farther removed 

 from the common Silurian parentage. Whatever the source of the 

 Oriskany fauna, it appears to have been developed outside the interior 



